1. Field of the Invention
This invention relates to a method for protecting copper and copper-based composites or monolithic structures from oxidation at elevated temperatures.
2. Description of the Prior Art
Copper alloys have generally superior thermal and electrical conductivity, especially pure copper or alloys having only minimal additions. Unfortunately, such materials also tend to oxidize rapidly at elevated temperatures, thus limiting their applicability for such purposes as high temperature heat exchangers, actively cooled gas flow channels, and air frame structures. For such applications, the alloys must have high strength and adequate oxidation resistance. Strengthening of copper alloys may be accomplished by compositing with a second metallic phase, such as niobium, tantalum, etc., known as microcompositing, or by reinforcement by high strength fibers, such as graphite.
Prior to the present invention, applicants have been unaware of any viable method to protect copper substrates for use at temperatures exceeding approximately 800.degree. F. It has recently been proposed, however, that copper and copper alloys, as well as copper-graphite fiber composites, be utilized for very high temperature heat exchange in rocket engines and hypersonic combustion engines. To achieve this, it was found necessary to provide a means to protect such materials from oxidation in the temperature ranges anticipated, e.g. 1200.degree. F. or higher. Moreover, for such utility, it was also necessary to provide protective coatings which would be compatible with the coefficient of thermal expansion of the substrate, and not substantially lower the thermal conductivity of the structure, while exhibiting fatigue resistance and resistance to diffusion.